ZFS read and write support has been included in the forthcoming Mac OS X Server. An announcement that in all the iPhone news went seemingly unnoticed.

Snow Leopard Server
>For business-critical server deployments, Snow Leopard Server adds read and write support for the high-performance, 128-bit ZFS file system, which includes advanced features such as storage pooling, data redundancy, automatic error correction, dynamic volume expansion, and snapshots.

While ZFS exists other places like FreeBSD, OpenSolaris etc, it's good to see a commercial product include support for one of the cooler things to come out of Sun. If you want to see how to take advantage of ZFS check out the links below, it's pretty much the same on every operating system excepting what the os calls your physical disks.

ZFS Basics - A look at ZFS from the OpenSolaris community.
ZFS Quickstart Guide - FreeBSD and ZFS.
ZFS on FUSE - Linux and ZFS.